Medical tests that require a small volume of blood are well known. For example, home test kits for measuring blood sugar levels are utilized by diabetics. These test kits require that a drop of blood be placed on a test strip that is then inserted into a measurement apparatus that displays the glucose concentration in the blood sample. To obtain the drop of blood, the user is supplied with a lancet device, which makes a skin prick, typically in the user's finger. Inserting the needle or lancet into the skin produces pain in the skin tissue. In patients such as diabetics, who have to sample their blood often, any excessive pain or tissue trauma is a disincentive to comply with the blood sampling routine. It would be advantageous to minimize the physical discomfort associated with this skin prick to obtain an amount of blood adequate for the test.
The skin is consisted of two layers--the epidermis and the dermis. The capillary structures connected to the arterial and venous vascular beds rise vertically and are located in the dermis layer. The neural sensors such as Meissner's corpuscles and free nerve endings are also located in the dermis. Layers of subcutaneous tissues lie below the dermis. The supply arterial and venous capillaries are located laterally in this tissue bed. There is also adipose tissue interleaved with afferent and efferent nerve fibers along with their associated sensors interwoven within the vascular bed. To successfully obtain blood, a piercing device such as a needle or lancet must traverse the skin's various layers to reach the blood vasculature. The amount of force necessary to penetrate the skin surface will depend on the force applied normal to the surface of the skin needed to exceed the rupture strength. There exists an elastic range within which the degree of deflection corresponds directly with the applied force (skin depression). When the rupture limit is exceeded, a non-linear response by the skin (otherwise known as the inelastic response) occurs, corresponding to the further stretching of the skin at the point of application prior to rupture. Bleeding occurs when the penetration of the object reaches the capillary bed.
A few techniques can be applied to reduce the pain sensation in blood sampling. One way is by minimizing the lancing angle of penetration, and hence minimizing the building of pressure waves at the penetration site. In addition, optimizing the depth of penetration by the lancet or needle in the skin reduces the sensation of pain. Optimizing factors such as the above to reduce patient discomfort will encourage compliance in self testing, as an example, for diabetic self-monitoring.
A successful method to minimize pressure waves generated in the puncture of the skin by a needle, pin or lancet, etc., would be to minimize the area over which the puncture occurs. This can be achieved by miniaturizing the needle or lancet, provided the force applied to create the wound is minimal. The smaller the needle, the less force is required to puncture the skin, and less nerves endings are stimulated by the puncture. One method for reducing the needle diameter and still providing an adequate amount of blood for a desired analysis is to use multiple lancets, each having a smaller diameter than the minimum needle diameter needed for a single needle prick that would yield the same amount of blood. Lancet or needle devices with multiple lancets (or needles) for sampling blood are disclosed in commonly assigned, copending patent application "Multiple lancet device," invented by Lum et al., Attorney Docket No. 10980684-1, filed on the same day as the present application, which is herein incorporated by reference in its entirety.
Currently available lancet devices do not incorporate a combination of lancets and blood transporting structures. Blood measurement instrumentation currently available from vendors typically contains only blood transporting and measurement structures. This requires the user to set up several devices in order to produce a blood measurement on a sample of blood. Such devices include the lancet, lancet-launching device, blood collection structures, and the blood analyte measurement module or system. Juggling the use of so many devices can severely hamper the user's interest in compliance of monitoring his/her blood chemistry. What is needed is a comprehensive blood sampling system that is capable of eliciting the blood and transporting it immediately (i.e., without delay by storing it first) and directly to the measurement and sensing area. This system will incorporate the necessary structures and reagents for measurement, starting from the lancet, the blood transport structures, the measurement area, to the interface with the instrumentation. Furthermore, since some analyte assays are sensitive to dilution by interstitial fluid, the blood that initially emerges from a lancing wound ("initial blood") tends to contain more of such interstitial fluid than later portions. What is needed is a blood sampling device that is able to draw off and discard a first portion of the elicited blood and then deliver the remainder to the measurement area. In other words, what is needed is a device that can deliver a blood sample more representative of the circulating blood in the blood vessels.
Patents of interest about blood sampling devices are, for example, U.S. Pat. No. 3,358,689; U.S. Pat. No. 4,469,110; U.S. Pat. No. 4,627,445; U.S. Pat. No. 4,837,274; U.S. Pat. No. 4,995,402; U.S. Pat. No. 5,047,044; and U.S. Pat. No. 5,314,442.